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Fish Kills and Strandings in Biscayne Bay

Following reports of fish kills in Biscayne Bay earlier this week, this morning the Pelican Harbor Seabird Station reported a strange sight: a large aggregation of Atlantic stingrays along the shallow edge of the bank on the north side of the Station, joined by numerous other species: checkered pufferfish, toadfish, spotted scorpionfish, butterfly rays, peacock flounder, and other normally bottom-dwelling species. It seemed as though these fish were trying to make their way to an area with more oxygen, some of them even seeming to “sip” air from the surface.

Based upon the photos that I received from my colleague at the Biscayne Bay Aquatic Preserves, it looked as though the fish were responding to low levels of dissolved oxygen in the water. Dissolved oxygen is critical for the health of fish, invertebrates and seagrasses, and if too low, conditions can become dangerous, and too high can also have consequences. Many factors affect dissolved oxygen including temperature, salinity, turbulence, nutrient inputs, and more.

Checkered pufferfish congregate in the shallow water on the north of end of the Pelican Harbor Seabird Station. Photo: Ana Zangroniz

This afternoon, I traveled to the Seabird Station, armed with equipment to test water temperature, pH, salinity, and dissolved oxygen. The average water temperature I found was 33 Celsius, or approximately 91 degrees F. Given that the water where I was measuring from was about 18 inches deep, this wasn’t unusual. What was interesting was that dissolved oxygen readings were less about 20’ from shore, and increased gradually as the readings were taken closer to shore, indicating that the stingrays and other fish had the right idea, they were following the oxygen. I’d like to note that these readings were preliminary and will still need to be finalized, but they indicate a phenomenon that’s opposite of what we would normally expect: deeper water=cooler temperatures=more available dissolved oxygen. However, that does not seem to be the case in this instance.

At this time, it is difficult to pinpoint an exact cause for these fish kills throughout the Bay. No one will doubt that the air temperatures have been stifling recently, but that isn’t unexpected for this time of year. Nutrients in Biscayne Bay over the past 15-20 years have created algal blooms and could be responsible for massive seagrass loss. While the early months of the COVID-19-related closures seemed to provide nature a break from human influences, the proliferation of human activity on the waterways could also be having the opposite effect.  At this point, it’s speculative at best, but could be considered as data is captured and analyzed by managers.

Many agencies are responding to this event: Miami-Dade County Department of Regulatory and Economic Services, Florida Department of Environmental Protection Biscayne Bay Aquatic Preserves, NOAA, UF/IFAS Extension, and others. In the meantime, if you spot fish kills or unusual fish aggregations, please report it to Miami Waterkeeper at hello@miamiwaterkeeper.org. Please stay tuned for updates on this blog as well as this program’s Facebook and Instagram accounts.